/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.Collections;

/**
 * GWT emulation of {@link SingletonImmutableBiMap}.
 *
 * @author Hayward Chan
 */
final class SingletonImmutableBiMap<K, V> extends ImmutableBiMap<K, V>
{

    // These references are used both by the custom field serializer, and by the
    // GWT compiler to infer the keys and values of the map that needs to be
    // serialized.
    //
    // Although they are non-final, they are package private and the reference is
    // never modified after a map is constructed.
    K singleKey;
    V singleValue;

    transient SingletonImmutableBiMap<V, K> inverse;

    SingletonImmutableBiMap(K key, V value)
    {
        super(Collections.singletonMap(checkNotNull(key), checkNotNull(value)));
        this.singleKey = key;
        this.singleValue = value;
    }

    private SingletonImmutableBiMap(K key, V value, SingletonImmutableBiMap<V, K> inverse)
    {
        super(Collections.singletonMap(checkNotNull(key), checkNotNull(value)));
        this.singleKey = key;
        this.singleValue = value;
        this.inverse = inverse;
    }

    @Override
    public ImmutableBiMap<V, K> inverse()
    {
        ImmutableBiMap<V, K> result = inverse;
        if (result == null)
        {
            return inverse = new SingletonImmutableBiMap<V, K>(singleValue, singleKey, this);
        }
        else
        {
            return result;
        }
    }

    @Override
    public ImmutableSet<V> values()
    {
        return ImmutableSet.of(singleValue);
    }
}
